1.4

• Draw the structure of a nucleotide


  Draw the structure of a nucleotide
• Pentose sugars in DNA & RNA

  • DNA: deoxyribose
  • RNA: ribose
• Describe how polynucleotide strands form


  Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone)
• Structure of DNA


  • Double helix of 2 deoxyribose polynucleotide strands (so there are 2 sugar-phosphate backbones)
  • H-bonds between complementary base pairs on opposite strands (AT & GC)
• Purine bases


  • adenine C5H5N5
  • guanine C5H5N5O
  • two-ring molecules
• Pyrimidine bases


  • thymine C5H6N2O2
  • cytosine C4H5N3O
  • uracil C4H4N2O2
  • one-ring molecules
• Complementary base pairs in DNA

  • 2 H-bonds between adenine (A) + thymine (T)
  • 3 H-bonds between guanine (G) + cytosine (C)
• Complementary base pairs in RNA

  • 2 H-bonds between adenine (A) + uracil (U)
  • 3 H-bonds between guanine (G) + cytosine (C)
• Semiconservative DNA replication


  • Strands from original DNA molecule act as templates
  • New DNA molecule contains 1 old strand & 1 new strand
• Role of DNA helicase in semiconservative replication


  Breaks H-bonds between base pairs to form 2 single strands, each of which can act as a template
• How a new strand is formed during semiconservative replication

  1. Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing

  2. DNA polymerase joins adjacent nucleotides on new strand in a 5' → 3' direction via condensation reactions to form phosphodiester bonds

  3. H-bonds reform
• Role of DNA ligase


  • Leading strand is replicated continuously in same direction as replication fork. Lagging strand is replicated in Okazaki fragments in the opposite direction
  • DNA ligase joins gaps in fragments to form a continuous strand
• Gene


  A sequence of bases on a DNA molecule that codes for a specific sequence of amino acids to make a polypeptide. Can also code for functional RNA
• Structure of messenger RNA (mRNA)


  • Long ribose polynucleotide with sugar-phosphate backbone
  • Nitrogenous bases: A, U, G, C
  • Single-stranded & linear (no H-bonds between complementary base pairs)
  • Codon sequence is complementary to exons of 1 gene from 1 DNA strand
• Structure of transfer RNA (tRNA)

  • Single strand folded into clover shape (some paired bases)
  • Anticodon on one end, amino acid binding site on the other
  • anticodon binds to complementary mRNA codon
  • amino acid corresponds to anticodon
• Transcription


  • Produces mRNA
  • Occurs in nucleus
• Outline the process of transcription

  1. RNA polymerase binds to promoter region on a gene

  2. Section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as template

  3. Free nucleotides are attracted to their complementary bases

  4. RNA polymerase joins adjacent nucleotides to form phosphodiester bonds
• What happens after a strand of mRNA is transcribed

  1. RNA polymerase detaches at terminator region

  2. H-bonds reform & DNA rewinds

  3. Splicing removes introns from pre-mRNA in eukaryotic cells

  4. mRNA moves out of nucleus via nuclear pore & attaches to ribosome
• Function of mRNA


  Transfers genetic code from DNA in nucleus to ribosomes for translation into a specific polypeptide
• Antisense strand of DNA


  Template strand of DNA which is transcribed
• mRNA


  Transfers genetic code from DNA in nucleus to ribosomes for translation into a specific polypeptide
• Sense strand of DNA

  Strand with the same base sequence as mRNA (but with thymine instead of uracil)
• Translation


  Produces proteins in the cytoplasm on ribosomes
• Translation


  1. Ribosome moves along mRNA until 'start' codon

  2. tRNA anticodon attaches to complementary bases on mRNA

  3. Condensation reactions between amino acids on tRNA form peptide bonds

  4. Process continues to form polypeptide chain until 'stop' codon is reached
• Role of ATP during translation

  ATP hydrolysis provides energy to form peptide bonds
• Features of the genetic code

  • Non-overlapping: each triplet is only read once
  • Degenerate: more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)
  • Universal: same bases and sequences used by all species
• DNA triplets


  Sequences of 3 bases that code for a particular amino acid
• Start codon


  Nucleotide triplet AUG on mRNA codes for the amino acid Met & initiates translation of a polypeptide
• Stop codon


  Nucleotide triplets on mRNA which do not code for an amino acid & terminate translation: UAA, UAG, UGA
• Exons and introns


  • Exons: regions of DNA that code for amino acid sequences. Separated by one or more introns
  • Introns: majority of DNA consists of non-coding regions within and between genes
• Mutation


  Any change in the base sequence of DNA. Often arise spontaneously during DNA replication
• Substitution mutation


  One nucleotide in the DNA sequence is replaced by another. This is more likely to be a silent mutation which does not change amino acid sequence
• Deletion mutation


  A nucleotide in the DNA sequence is lost, leading to a frame shift. Significant since entire amino acid sequence downstream of mutation will be different
• Insertion mutation

  Addition of one or more base pairs to DNA sequence, often in microsatellite regions. Causes frameshift. Significant since entire amino acid sequence downstream of mutation will be different
• Sickle cell anaemia


  Genetic condition that results in abnormal haemoglobin. Impaired ability to transport oxygen = rapid heart rate, fatigue, dizziness. Sickle shaped red blood cells 'stick' in vessels
• Sickle cell anaemia in humans is caused by a missense point mutation in the gene that codes for the β strand in haemoglobin. On DNA: CTC (Glut) → CAC (Val). Change in primary structure = different tertiary structure. Abnormal haemoglobin molecules form strands that make red blood cells sickle shaped.